The present invention relates to improvement of a single focus imaging lens to form an image of a photographic subject in various cameras including a so-called silver-halide camera, in particular, in a digital camera, a video camera, and the like. In particular, the present invention relates to an imaging lens that is suitable as an imaging optical system in an imaging apparatus using an electronic imaging device as a digital camera or a digital video camera, an imaging apparatus such as a camera, or the like using such an imaging lens, and an information device such as a portable information terminal device, or the like having an imaging function.
The market for so-called digital cameras or the like, which obtain digital image data of a photographic subject using a solid-state image sensor, has grown dramatically, and there have been various requests from users regarding such digital cameras, or the like.
Above all, the category of compact cameras with high picture quality, which include a relatively large image sensor having a diagonal length of approximately 20-45 mm and an imaging lens of a high-performance single focus optical system, has gained a great deal of attention.
As requests from users, in addition to high performance, emphasis has been put on excellent portability, that is, miniaturization.
Here, in terms of high performance, in addition to having resolution corresponding to an image sensor of at least 12-24 mega pixels, less comatic flare, high contrast, and no point image distortion in a peripheral part of an angle of view at an open aperture, less chromatic aberration and no occurrence of unnecessary coloring in a part having a large brightness difference, less distortion, and capability of drawing a straight line as a straight line, and so on are at least needed.
In terms of the large diameter, due to the necessity to make different from a general compact camera with a zoom lens, an F-number of at least approximately F2.8 is needed.
In terms of miniaturization, in a high-resolution compact camera, since a large image sensor is used relative to the size of the camera body, an actual focus length is longer than that of a compact camera having a small image sensor. Therefore, in a case of normalizing the camera by a focal length or a maximum image height, it is necessary to shorten the entire length of the imaging lens in the camera.
Additionally, in terms of an angle of view of a photographing lens, many users have demanded a rather wide angle, and it is preferable that a half angle of view of an imaging lens be equal to or more than 35 degrees. The half angle of view of 35 degrees is equivalent to a focal length of about 31 mm in a 35-mm silver-halide camera using traditional 35-mm silver-halide film (so-called Leica format silver-halide film).
As an imaging lens using this kind of digital camera or the like, many kinds of optical systems have been considered; however, as a construction of a major wide-angle single focus lens, a so-called retrofocus-type optical system in which a lens group having a negative refractive power is arranged on an object side, and a lens group having a positive refractive power is arranged on an image side is included.
The main reason why the retrofocus-type optical system is adopted is that there is a request that a position of an exit pupil be distant from an imaging plane, and peripheral light flux be incident onto an area sensor at an angle of preferably approximately a right angle from a characteristic of the area sensor as an image sensor having a color filter and a microlens per each pixel.
However, in the retrofocus-type optical system, a wide-angle lens is used as an interchangeable lens for a single reflex camera, as is clear from adaption of the retrofocus-type optical system for the purpose of securement of a back focal distance, and therefore, an entire lens length (a distance from a surface on a most object side to an image plane) tends to be longer.
On the other hand, in recent years, in a relatively large image sensor having a diagonal length of 20-45 mm, due to improvement and optimization of an on-chip microlens, development of image processing, and the like, a slightly inclined incident state has been allowable as a state where peripheral light flux is incident onto the area sensor.
In particular, a system capable of sufficiently accepting an angle between a principal ray and an optical axis at a maximum image height of about 35-45 degrees is constructible, and it is possible to choose types of lenses more suitable for miniaturization without restricting a condition of vertical incidence of peripheral light flux.
From the above viewpoint, as types of optical systems more suitable for miniaturization than the retrofocus-type optical system, there are an approximately symmetrical-type, and a telephoto-type in which a lens group having a negative refractive power is arranged on an image side. Conventional examples of imaging lenses of this kind are disclosed in Japanese Patent Application publication Numbers H08-313802, H11-326756, 2005-352060, 2012-008347, and the like.